Fix crash on Python < 3.14 when paramiko is not installed

The return annotation of _ssh_connect referenced paramiko, which
Python < 3.14 evaluates at class definition, raising NameError when
paramiko (an optional dependency) is missing. Postpone evaluation
of annotations, and mark the return type as Optional.

simple_backup/simple_backup.py

@@ -11,6 +11,9 @@ Classes:
     Backup
 """
 
+# Avoid evaluating annotations at definition time, as they reference optional modules (paramiko)
+from __future__ import annotations
+
 # Import libraries
 import traceback
 import sys
@@ -327,7 +330,7 @@ class Backup:
             except IndexError:
                 logger.info('No previous backups available')
 
-    def _ssh_connect(self, homedir: str = '') -> paramiko.client.SSHClient:
+    def _ssh_connect(self, homedir: str = '') -> Optional[paramiko.client.SSHClient]:
         try:
             ssh = paramiko.SSHClient()
         except NameError: